The SS may full well know that he is there but they cant see him because he is using the front of the building like a merlon.
The SS cant just start hosing down a building randomly because someone says there is someone else on the roof. They have to see him to aim which they cant do until he pops up.
This could explain why the sharpshooters had to wait until the perp started shooting. They weren't in a position to see him until then.
Still doesn't explain why no drones though.
Complete drone “grids” are possible for such events and for large area security. Not having them for this is a FAIL or intentional.